My invention concerns a system for recording information, such as audio signals, on magnetic record media notably including magnetic tape. More specifically, my invention deals with such a magnetic recording system incorporating the concept of variable, signal-controlled recording bias and equalization to suit the spectrum of the information signal being recorded.
The magnetic recording of information on tape or like recording media requires the feeding of a high-frequency signal, termed a bias, to the recording head together with the information signal being recorded, for the reduction of distortion. The information signal must further undergo a corrective process called equalization, and must be fed to the recording head at a suitable level, to be properly recorded on magnetic tape or the like.
The magnitude of the high frequency bias current had long been fixed at a value chosen in view of the expected relative levels of the low and high frequencies of the input spectrum. The choice was a compromise one, however. The conventional fixed values of bias were usually too high for high frequencies, inhibiting their faithful recording or playback. Thus the tape decks or similar devices built on the fixed bias principle did not necessarily satisfy the discriminating ears of audiophiles.
A solution to the above problem is the Dolby headroom extension (HX) system proposed by Kenneth Gundry, of Dolby Laboratories, Inc., at the 64th convention of the Audio Engineering Society (AES) held in November, 1979, in New York City (available in written form as an AES preprint entitled "Headroom Extension for Slow-Speed Magnetic Recording of Audio"). The Dolby HX system (shown in FIG. 1 of the attached drawings) includes a control circuit for reducing bias in response to high levels at high frequencies of the program being recorded. The reduced bias makes it possible to make an accurate recording of the high frequencies without sacrifice of performance of low frequencies.
Admittedly, the Dolby HX system enables the addition of nearly optimum bias to the information signal of varying frequencies from moment to moment. It does, however, possess its own drawbacks.
Both channels of a stereophonic magnetic recording system according to the Dolby HX scheme share one and the same bias oscillator and its control means. With changes in bias voltage applied to the bias oscillator, therefore, corresponding variations in bias current occur in both channels. This presents no problem at all as long as the signals on the two channels have the same frequencies and the same levels. A problem arises when, as is usually the case with audio signals, the signal frequencies and levels differ between the two channels. An obvious solution is to provide a different bias oscillator, as well as associated control means, for each channel, but this will make the circuit configuration unnecessarily complex.
Another objection to the Dolby HX system comes from the fact that the bias oscillator must also deliver its output to the erase head or heads. Since the bias changes take place at the bias oscillator itself, the magnitude of the erase signal varies inevitably. Thus the erase head or heads will not offer a uniform erasing function.